Dark and light adaptation probably do not reflect a single simple process. The aim of this research is to study such mechanisms as appear to be separable for individual comparisons with visual sensitivity. A device has been constructed to permit threshold measurements and visual pigment measurements with the same apparatus, hence, with the same visual parameters. The device provides graphical recording of the pigment changes and maintains an infrared reference signal on the retina at the locus of the pigment signal. Results indicate that the return of sensitivity during dark adaptation may not always follow the course of recovery of the visual pigment after complete bleaches, as is thought sensitivity may instead return faster, as it is already known to return faster following slight bleaches. Very early dark adaptation probably reflects a sensitivity-limiting mechanism separate from, and additional to, the mechanisms that recover during regular/slow dark adaptation. The distortion introduced by ignoring the early mechanism in prior estimates of "Equivalent Background" during dark adaptation has been measured and is found to be as much as a fractor of 50 in extreme cases, reducing to zero near absolute threshold levels for rods and for cones.